Types and expressions

pyFlies has a type system with a some usual types you can find in other languages, but also some types that are useful in the context of experiments specification. Operations on these types can form complex expressions. You can use expressions in various places of your experiment specification.

Types

pyFlies values can be of the following types:

  • bool - with values of true and false. You will rarely need to use these values directly. They are usually produced as a results of boolean expression evaluation. For example, an expression
    direction == right

will have a bool value

  • int - is an integer number (e.g. 42)
  • float - is a float number (e.g. 3.44)
  • point - specifies a location in two dimensional space (e.g. (2, 5))
  • color - specifies a color in a standard CSS like notation where we have a # prefix and 6 hex digits, 3 groups of 2 digit where each group represents red, green and blue component of the color (e.g. #455a45).
  • string - a string of characters enclosed with quotes (e.g. "Hello"). Strings content is piped through Jinja template engine during evaluation so a powerful string interpolation is available. This interpolation is also done for screens.
  • list - a list of values of any type (e.g. [1, 2, "hi", [true, 2.3], left])
  • range - is a shorthand for definition of a list with consecutive numbers (e.g. 1..5). Usually used for looping in table expansion, or producing a random value from a given range (e.g. 1..100 choose).

  • symbol - is just a word which might get bound to a value if the variable of the same name is available in the context. If not, a symbol can be mapped to a value by the compiler. Usually used to express abstract terms. For example:

    directions = [left, right]

    Here, values in the directions list are symbols whose meaning may be determined by variables or by the compiler.

Tip

When editing colors in the experiment specification in VS Code editor, various extension from the VS Code marketplace may help. For example Color Manager.

Expressions

if

An expression used to provide a value that depend on a condition. For example:

left if cue_pos == right else right

So, the form is always:

<value if true> if <condition> else <value if false>

loop

loop expression is used in table expansion. Its form is:

<expression> loop

where expression evaluates to a list or range.

For example:

[left, right, up, down] loop

Logic

Logic operators are: or, not, and and. or and and are standard infix binary operators while not is a standard unary prefix operator:

color == blue and not practice

Comparison

Comparison operators can compare values. They are used as standard infix binary operators. Following operators are supported:

  • == - equal
  • != - not equal
  • <= - less or equals than
  • >= - greater or equals than
  • > - greater than
  • < - less than

Arithmetic operators

Standard arithmetic operators are supported: +, -, *, / and they work as usual.

Randomization

Randomization is supported with choose and shuffle operations. These operations are given in a postfix form and are applicable only to lists and ranges.

  • choose will choose a random element from list/range. For example: 

    [red, green, blue] choose
1..100 choose

  • shuffle will produce a list where elements are in a random order. For example:

    1..100 shuffle

In the current implementation of the compiler all expressions are pre-evaluated by the compilation process. E.g. random values are predetermined at compile time, not run-time. This is done to make generator development easier as otherwise compiler would need to translate all expressions to the target language expressions. This is an implementation detail that might change in the future.